Added Markdown CSS

I've added the [CSS used by Github](https://github.com/sindresorhus/github-markdown-css) to render README.md files to the registry. 

I cannot confirm if this compiles, as I am getting a strange error when I run `dub`:

```
Error executing command run: Root package dub-registry contains reference to invalid package dub
```

However, I have made the edits with the Firefox Dev Tools and everything looks good.

![gh-readme](https://cloud.githubusercontent.com/assets/512416/4530532/a1a70822-4d81-11e4-80ac-2b9cd8e0f53a.PNG)

P.S. You should add documentation about [Blocks](http://vibed.org/templates/diet) for Diet templates.